1. Field of the Invention
The present invention relates to a polymer scale preventive agent for use in polymerization of a monomer having an ethylenically unsaturated double bond, and to a polymerization vessel and a process for producing a polymer which utilize the polymer scale preventive agent.
2. Description of the Prior Art
Heretofore, methods for polymerization of a monomer having an ethylenically unsaturated double bond have been known, such as suspension, emulsion, solution, gas phase and bulk polymerization processes and the like. In any of these polymerization processes, polymer scale is liable to be deposited on the areas with which the monomer comes into contact, such as inner walls, stirring equipment and so on of a polymerization vessel.
The deposition of the polymer scale results in disadvantages that the yield of the polymer and cooling capacity of the polymerization vessel are lowered, and that the polymer scale may peel off and mix into a polymeric product, thereby impairing the quality of formed products obtained by processing the polymeric product. In addition, removal of deposited polymer scale is very laborious and time-consuming. Further, the polymer scale contains unreacted monomers and, therefore, may cause physical disorders in the operators, which has been a very serious problem in recent years.
For preventing polymer scale deposition on the polymerization vessel inner wall and so forth, methods have been known, for example, a method in which a polymer scale preventive agent comprising a polar organic compound such as amine compounds, quinone compounds, aldehyde compounds, etc. is applied to the polymerization vessel inner wall and so on to form a coating and a method in which such compounds are added to an aqueous medium (Japanese Patent Publication (KOKOKU) No. 45-30343 (1960)), as practiced in some instances of suspension polymerization of vinyl chloride.
However, these methods have the disadvantage that, although the polymer scale preventive effect is exhibited while polymerization is repeated for up to about 5 or 6 batches, the effect diminishes if the number of repeated batches of polymerization exceeds 5 or 6 (that is, the scale preventive effect is poor in durability). The disadvantage is emphasized particularly where a water-soluble catalyst is used for polymerization, and, in this point, the polymer scale prevention according to these methods is unsatisfactory industrially.
For overcoming the above disadvantage, methods have been proposed in which the inner wall of a polymerization vessel is coated with a polymer scale preventive agent whose effective constituent is, for example, a condensation product of an aromatic amine compound and an aromatic nitro compound (Japanese Patent Publication (KOKOKU) No. 60-30681 (1985)), a reaction product of a phenolic compound with an aromatic aldehyde (Japanese Pre-examination Patent Publication (KOKAI) No. 57-192414 (1982)), a reaction product of a polyhydric phenol with an aliphatic aldehyde (Japanese Patent Publication (KOHYO) No. 57-502169 (1982)), a reaction product of 1-naphthol with formaldehyde (Japanese Patent Publication (KOKOKU) No. 01-31523 (1989)), or the like. Where one of these known polymer scale preventive agents is applied to the areas with which monomers come into contact, such as the inner wall surface of a polymerization vessel, and a coating is thereby formed, it is possible to repeat about 100 to 200 batches of polymerization without causing deposition of polymer scale on the areas located in the liquid-phase region inside the polymerization vessel. Besides, even in the above-mentioned case where a water-soluble catalyst is used, deposition of polymer scale in the liquid-phase region is similarly prevented.
However, there still remains a drawback that polymer scale deposition may occur on the areas around the interface between the liquid phase and the gas phase located at an upper portion of the interior of the polymerization vessel.
Once polymer scale deposition occurs the areas around the interface between the gas and liquid phases, the deposited scale will grow gradually as polymerization runs are repeated, and at last it may peel off to be incorporated into the polymeric product. If the polymeric product with the polymer scale thus mixed therein is processed into formed products such as sheets or the like, the polymer scale causes generation of many fish eyes in the formed products, thereby lowering seriously the quality of the formed products.
Besides, the polymeric product obtained upon polymerization is required to have a high whiteness. That is, when a polymeric product is formed into a sheet or the like without any addition of a coloring agent, the resulting formed product is more or less colored and such coloration, called initial coloration, is desired to be as slight as possible. However, the coating which contains a condensation product of an aromatic amine compound, etc. as mentioned above may peel off or dissolve in the polymerization mass, to be incorporated into the polymeric product, whereby the resulting formed products have poorer quality as to initial coloration.